The present invention relates to a filter typically employed in mobile communications equipment, such as a mobile phone.
With the increasing use of a mobile phone and other mobile communications in recent years, a demand for more compact and inexpensive mobile communications equipment including mobile phones has now been growing. To serve the demands, it is essential to form each electric circuit as a component of such equipment to be smaller and lower in cost. It has been difficult, however, to structure a radio-frequency circuit section compact because of a filter that is hard to be integrated into one chip. Therefore, miniaturization and integration of each circuit component in a radio-frequency circuit section will be a key factor in structuring the equipment compact and inexpensive.
FIG. 6 is a block diagram depicting a typical radio-frequency circuit section of prior-art mobile communications equipment. In FIG. 6, an radio frequency transmission signal generated at IC 601 is fed into band-pass filter 603 via impedance transformer 602. Impedance transformer 602 is responsible for matching the impedance of IC 601 to that of transmission band-pass filter 603. After passing through band-pass filter 603, the radio frequency transmission signal is power-amplified by amplifier 604. After that, the signal goes through duplexer 605 then radiates from antenna 607. Impedance transformer 602 above may be the type that splits the signal path into two branches according to the specifications of IC 601.
In either case, impedance transformer 602 and band-pass filter 603 are separately structured as an independent circuit component, taking up too much space in the circuit. The structural limitations have therefore been an obstacle to more downsized and inexpensive equipment using such components.
The present invention addresses the problem above. It is therefore an object of the present invention to provide a downsized filter by integrating a band-pass filter with an impedance transformer into a simple structure with the help of electromagnetic coupling between parallel resonators.
The filter disclosed in the present invention is a parallel-resonance type band-pass filter, which includes a substrate, a first resonator, and a second resonator.
The first resonator includes a capacitor and a plurality of connected-in-series inductors, both of which are formed on the surface or an inner layer of a substrate.
Similarly, the second resonator includes a capacitor and a plurality of connected-in-series inductors, both of which are formed on the surface or an inner layer of a substrate.
The electromagnetic coupling between the first and second resonators is established by at least the electromagnetic coupling between one of the inductors of the first resonator and one of the inductors of the second resonator.
The present invention has various aspects described below:
(1) inductors for each resonator may be three or more connected-in-series inductors. In this case, flexibility in designing a resonator will be increased.
(2) a capacitor for each resonator may be an inter-digital type capacitor. This will realize a capacitor formed on a single layer substrate, allowing the filter to have a low profile.
(3) employing a balanced-type terminal for at least one of the input terminal and the output terminal, and connecting each terminal of the balanced-type terminal with a resonator via a capacitor. This will realize a balanced-type, 2-stage parallel-resonance type band-bass filter
(4) grounding the middle point of the connected inductors in the resonator will eliminate unstable operations occurred at the grounded position in frequencies of the microwave-frequency band or higher.
(5) making a difference between the input impedance and the output impedance of a filter allows the filter to also serve as an impedance transformer as well.
(6) employing a dielectric material for the substrate will realize a smaller filter.
(7) employing a semiconductor wafer for the substrate allows a filter not only to be compact, but also to integrate with other semiconductor parts onto an IC chip.
(8) aforementioned semiconductor may be: i)silicon, ii) gallium arsenide, iii) silicon-germanium, iv) indium phosphide, or v) a compound having any one of elements above i) through iv) as a major constituent. Such formed filter will be able to exploit each own advantage according to a use.
(9) aforementioned inductors may be formed by an intaglio-printing technique or a thin-film forming technique to form the filter compact.
(10) aforementioned capacitors may be formed by a thin film-, or a thick film-forming techniques to form the filter compact.
(11) the filter electrodes may be made of: i) copper, ii) silver, or iii) a metal compound having one of copper and silver as a major constituent. This contributes to a strength-increased substrate, or simplified manufacturing steps.
With such structures described above, according to the present invention, it is possible to form a filter not only determining the degree of coupling with flexibility between the resonators, but also determining an input impedance so as to be different from an output impedance. This advantage realizes a band-pass filter that doubles as an impedance transformer, shrinking the physical size of mobile communications equipment using the filter.